1. Field of the Invention
The present invention relates to a topcoat composition for use in the process of producing a semiconductor such as IC, in the production of a circuit board for liquid crystal, thermal head and the like, and in the lithography process of other photofabrication; an alkali developer-soluble topcoat film using the composition; and a pattern forming method using the topcoat film. More specifically, the present invention relates to a topcoat composition suitable for exposure by an immersion-type projection exposure apparatus using a light source that emits far ultraviolet light at a wavelength of 300 nm or less; an alkali developer-soluble topcoat film using the composition; and a pattern forming method using the topcoat film.
2. Description of the Related Art
Along with the finer fabrication of a semiconductor device, there is becoming shorter the wavelength of the exposure light source and higher the numerical aperture (high NA) of the projection lens, and an exposure machine with NA of 0.84 using an ArF excimer laser having a wavelength of 193 nm as a light source has been so far developed. As commonly well known, these features can be expressed by the following formulae:(Resolving power)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
In order to realize still shorter wavelength and higher resolving power, studies are being made on an exposure machine where an F2 excimer laser having a wavelength of 157 nm is used as the light source. However, the lens material used for the exposure apparatus so as to realize shorter wavelength and the material used for the resist are very limited and therefore, it is extremely difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in outfitting the exposure apparatus and the resist each assured of sufficiently high performance and stability within a required time period.
Conventionally, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been known as a technique of increasing the resolving power in an optical microscope.
As for the “effect of immersion”, assuming that λ0 denotes the wavelength of exposure light in air, n denotes the refractive index of the immersion liquid to air and θ denotes the convergence half-angle of beam and that NA0=sin θ, the above-described resolving power and depth of focus when immersed can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the depth of focus can be made n times larger by the immersion.
This is effective for all pattern profiles and can be combined with super-resolution techniques which are being studied at present, such as phase-shift method and modified illumination method.
Examples of the apparatus where this effect is applied to the transfer of a fine image pattern of a semiconductor device are described in JP-A-57-153433 and JP-A-7-220990 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
JP-A-10-303114 indicates that the control of refractive index of the immersion liquid is important because a change in the refractive index of the immersion liquid brings about deterioration of the projected image due to wave front aberration of the exposure machine, and discloses an immersion liquid with a refractive index of which the temperature coefficient is controlled to a certain range, or, as a suitable immersion liquid, water where the surface tension is decreased or an additive of increasing the surface activity is added. However, the additive is not disclosed or the resist suitable for immersion exposure techniques is not discussed.
Recent progress of the immersion exposure technique is reported, for example, in SPIE Proc., 4688, 11 (2002) and J. Vac. Sci. Tecnol. B, 17 (1999). In the case of using an ArF excimer laser as the light source, in view of safety on handling as well as transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered most promising as the immersion liquid.
In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in view of balance between transmittance and refractive index at 157 nm, but those satisfied from the aspect of environmental safety or refractive index have been not yet found out. The immersion exposure technique is expected to be most soon mounted on an ArF exposing machine in view of the degree of immersion effect and the maturity of resist.
Since the advent of a resist for a KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for a resist so as to compensate the reduction in the sensitivity due to light absorption. To describe this by taking as an example the image forming method using positive chemical amplification, an acid generator in the exposed area decomposes upon exposure to generate an acid, the acid generated is used as a reaction catalyst in the post-exposure baking (PEB) to convert the alkali-insoluble group into an alkali-soluble group, and the exposed area is removed by alkali development.
In the immersion exposure, the resist film is exposed through a photomask in the state of an immersion liquid being filled between the resist film and the optical lens, and the pattern of the photomask is thereby transferred to the resist film. At this times there is a case where an image is not formed due to permeation of the immersion liquid into the inside of the resist film (Nikkei Micro-Device, April, 2004). Also, it is feared that due to elution of an organic substance or the like from the resist film into the immersion liquid, impurities are mixed in the immersion liquid and contaminate the lens or exposure apparatus to hinder the exposure.
In order to avoid such a problem, a method of providing a topcoat film between the resist film and the lens to prevent the resist from coming into direct contact with water is known (see, for example, Nikkei Micro-Device, April, 2004).
It is not clarified yet what a material is suitable for the topcoat, but studies by the present inventors reveal that depending on the material of the topcoat, bad coating uniformity on the resist film or generation of particles after aging or storage may occur. In this respect, improvement is necessary. Also, there is room for improvement on the sensitivity of the underlying resist film.